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Extrusion process of 304L H-shaped stainless steel used in passiveresidual heat removal heat exchanger
Lei-Feng Tuo1,2 • Gen-Shu Zhou1 • Zhi-Qiang Yu1 • Xi-Tang Kang2 •
Bo-Wen Wang2
Received: 11 July 2018 / Revised: 2 October 2018 / Accepted: 20 October 2018 / Published online: 13 March 2019
� China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, Chinese
Nuclear Society and Springer Nature Singapore Pte Ltd. 2019
Abstract 304L H-shaped stainless steel is used as the
support frame of the passive residual heat removal heat
exchanger (PRHR HX) in a nuclear fission reactor. The
extrusion process is adopted to manufacture the 304L
H-shaped stainless steel. Finite element method simulation
is herein used to analyze metal flow characteristics, opti-
mize the extrusion die, and predict the extrusion force at
different temperatures and speeds. A U400-mm container
and U388-mm forging billet are selected, and the 304L
H-shaped stainless steel is successfully manufactured using
a Germany SMS 60 MN horizontal extruder. The
mechanical properties and microstructure of the manufac-
tured 304L H-shaped stainless steel meet the requirements
of the PRHR HX, and the surfaces of the product pass the
dye penetration test. The H-shaped stainless steels are used
in Haiyang nuclear power plant in Shandong Province.
Keywords PRHR HX � Support frame � Extrusionprocess � 304L � H-shaped stainless steel
1 Introduction
The passive safe system, which improves the safety and
reliability of nuclear power plants, is generally used in
third-generation nuclear technology. Current third-genera-
tion nuclear technology includes American AP1000 [1],
European EPR [2], Chinese HPR1000 [3], etc. The passive
residual heat removal heat exchanger [4] (PRHR HX) is a
crucial piece of equipment that allows for passive cooling.
When a power plant stops working in the event of a sudden
accident such as a natural disaster, the PRHR HX rapidly
discharges the residual heat to the external environment,
which is important for the entire system as well as for the
safety of the staff. Figure 1 shows the structures of the
PRHR HX and its support frame.
Many stainless steels are used in the safety systems of
nuclear power plants. Austenitic stainless steels are used
in lead–bismuth eutectic (LBE) loops for integrated tests
of operability and safety of proliferation-resistant, envi-
ronment-friendly, accident-tolerant, continual, and eco-
nomical reactors [5]. AISI 316L stainless steel pipes are
used in lead–bismuth eutectic (LBE) loops for integrated
tests of operability and safety [6]. 316LN is used as
AP1000 primary coolant pipes [7]. H-shaped stainless
steels are used in nuclear fission reactors as the support
frames of residual heat removal exchangers, as shown in
Fig. 2.
H-shaped stainless steels used in PRHR HX are gener-
ally manufactured by a welding process. Cai [8, 9] studied
the manufacture of H-shaped stainless steel used as the
support for AP1000 passive residual heat removal heat
exchangers. The H-shaped stainless steel is first welded
using three plates and then machined to its final size.
Severe welding distortion occurs because residual stress
This work was supported by the State Key Laboratory for Mechanical
Behavior of Materials (No. 20171909).
& Gen-Shu Zhou
1 State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, China
2 Stainless Steel Tube Branch Company, Taiyuan Iron & Steel
(Group) CO. LTD, Taiyuan 030003, China
123
NUCL SCI TECH (2019) 30:61(0123456789().,-volV)(0123456789().,-volV)
https://doi.org/10.1007/s41365-019-0591-5
exists in the H-shaped stainless steel after the welding
process. Moreover, the plate cutting utilization efficiency
and manufacturing efficiency are low. Wang [10, 11]
studied the manufacturing process of the support frame of
the PRHR HX. TP304 H-shaped stainless steel is also
manufactured by a welding process, and it is very difficult
to control the welding distortion. Besides, the welds in
H-shaped stainless steel increase the risk to the PRHR HX,
so it is necessary to develop another direct forming process
(seamless H-shaped stainless steel) to replace the welding
process.
In contrast to the welding process, the extrusion process
avoids the welds, the performance of the product obtained
is consistent, and the cost of the process is low. Moreover,
three-dimensional compressive stress exists in the extru-
sion deformation process, the structure is homogeneous,
the dimensions of the H-shaped stainless steel are accurate,
and the surfaces of the H-shaped stainless steel are good
because of glass lubrication.
In order to meet the requirements of the support frame
of the residual heat removal exchanger in the Haiyang
nuclear power plant in Shandong province, China, the
extrusion process of seamless 304L H-shaped stainless
steel is herein studied.
2 Materials and methods
2.1 Material
The billet material is 304L stainless steel, which is
manufactured using an ingot casting, radial forging, and
machining process. Table 1 shows the key elemental con-
stituents; the main alloy elements are Cr and Ni.
2.2 Extrusion model
304L H-shaped stainless steel is manufactured using an
SMS 60 MN horizontal extruder. The extrusion model
consists of an extrusion stem, container, block, billet, glass
pad, and mold system, as shown in Fig. 3a. The mold
system includes a die, cushion, and bolster, as shown in
Fig. 3b. The cushion is also designed with an H-shaped
hole, which is a key component to withstand the large
extrusion force. Figure 3c shows a flat-die and a cone-die.
The material of the die, cushion, and bolster is 4Cr5Mo-
SiV1 (GB/T1299-2000).
2.3 Deformation design
The internal diameter of the extrusion container ‘D’ is
U400 mm. The thermal expansion coefficient ‘l’ is about1.02–1.03 when austenite stainless steel is heated to 1373–
1573 K. The thickness of the glass powder layer ‘h’ at the
external surface of the 304L stainless steel billet is typically
0.5–1 mm. The diameter of the billet is smaller than the
internal diameter of the container; the final diameter of the
billet ‘D1’ is U388 mm [Eq. (1)]. The final sectional
dimensions of the 304L H-shaped stainless steel are
H211.5 mm 9 229 mm 9 15.5 mm 9 25 mm. The extru-
sion ratio ‘k’ of the 304L H-shaped stainless steel is 9.4
[Eq. (2)]. ‘SC’ is the sectional area of the internal diameter of
the container, ‘SH’ is the sectional area of the H-shaped
stainless steel. Figure 4a, b shows the design drawings of the
billet and the H-shaped stainless steel.
D ¼ ðD1 � 2hÞ=l ð1Þk ¼ SC=SH ð2Þ
2.4 Heating process
In order to uniformly heat the 304L stainless steel billet
uniformly, a circular furnace (Capital Engineering &
Fig. 1 Passive residual heat removal heat exchanger
Fig. 2 H-shaped stainless steel in a passive residual heat removal
heat exchanger
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61 Page 2 of 10 L.-F. Tuo et al.
Research Incorporation Limited, China) (Fig. 5a) and an
industrial frequency induction furnace (Inductotherm
Banyard, USA) Fig. 5b are adopted.
2.5 Lubricant and lubrication method
In the hot extrusion process of stainless steel, glass
lubrication serves to provide thermal insulation as well as
Table 1 Key elemental
constituents of 304L stainless
steel
Element C Si Mn P S Cr Ni N Co
% 0.03 0.27 2 0.018 0.001 18.22 8.15 0.06 0.02
Fig. 3 (Color online)
a Extrusion model, b mold
system, and c flat-die and cone-
die
Fig. 4 a Billet dimensions and
b H-shaped stainless steel
dimensions
Fig. 5 (Color online) a Circularfurnace and b industrial
frequency induction furnace
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Extrusion process of 304L H-shaped stainless steel used in passive residual heat removal heat… Page 3 of 10 61
to reduce friction. Hansson [12] analyzed a finite element
model of glass-lubricated extrusion of stainless steel tubes.
Glass powder and glass pads are typically used as lubri-
cants for stainless steel. Glass powder is used to lubricate
the interface between the billet and the container; the billet
will roll at the glass powder lubrication platform before
extrusion begins. A glass pad is used to lubricate the
extrusion die. Before flowing out the die hole, the metal
must break the glass pad. The glass pad retards the defor-
mation time significantly, in order to reduce the impact of
the glass pad, an H-shaped glass pad is fabricated.
2.6 Simulation boundary conditions
In order to research the extrusion deformation process
and predict the extrusion force of 304L H-shaped stainless
steel, FEM simulation is used. The simulation process
includes the definition of the materials, establishment of
the model, and definition of the boundary conditions. 304L
stainless steel in the material library of DEFORM-3D is
used to define the billet material. Moreover, 4Cr5MoSiV1
is used to define the extrusion die, extrusion container, and
ram.
Because glass powder and a glass pad are used as
lubricants, the friction coefficient is set manually. Liu [13]
studied the lubrication behavior of the glass-lubricated hot
extrusion process and found the friction coefficient to be
0.03. The extrusion speed of the ram is defined between
150 mm/s and 400 mm/s, the extrusion temperature is
defined between 1453 K and 1493 K, the finite mesh ele-
ment is tetrahedral in shape, the solution step is defined
with a time increment of 0.5 s, the primary die is ram, the
solver is a conjugate-gradient, and the iteration method is
direct iteration.
3 Results and discussion
3.1 Simulation results
3.1.1 Metal flow characteristics
The metal flow characteristics of the flat-die and cone-
die are different, as shown in Fig. 6. The metal flow of the
flat-die is directed from the circle edge to the H-shaped die
hole, but the metal flow of the cone-die obviously shunts
from zone ‘A’ to zones ‘B’ and ‘C’. In zones ‘B’ and ‘C’,
the directions of metal flow are ‘Y’ and ‘X’, which are
perpendicular to the edges of the H-shaped die hole.
The transitive characteristics of the forces on the flat-die
and cone-die are different. In the flat-die, the directions of
forces ‘FA’, ‘FB’, and ‘FC’ are perpendicular to the upper
surface of the flat-die, as shown in Fig. 7a. In the cone-die,
parts of the forces ‘FA’ and ‘FC’ are decomposed into ‘F1’
and ‘F2’, while the other parts of the forces ‘FA’ and ‘FC’
are perpendicular to the upper surface of the flat-die. ‘FB’
is decomposed into ‘F1’ and ‘F2’, as shown in Fig. 7b. The
decomposition of force reduces the extrusion force and
effectively improves the service life of the die.
Five points, namely, ‘P1’, ‘P2’, ‘P3’, ‘P4’, and ‘P5’, are
selected to track stress, strain, strain rate and velocity
distribution during the extrusion process. ‘P1’, ‘P2’, and
‘P3’ are located in the formed H-shape stainless steel, and
‘P4’ and ‘P5’ are located in the unformed billet. The
locations of the five points are shown in Fig. 8.
Figure 9a, b shows the strain change trend; the maxi-
mum strain of ‘P4’ and ‘P5’ is about 4.5, while that of ‘P1’,
‘P2’, and ‘P3’ is about 1.5. Figure 9c, d shows the strain
rate change trend; the maximum strain rate of the flat-die is
5 s-1 larger than that of the cone-die. Figure 9e, f shows
the stress change trends of the flat-die and cone-die. The
metal at ‘P1’, ‘P2’ and ‘P3’ flows almost synchronously,
and deformation is completed in 0.5 s. ‘P4’ and ‘P5’ are at
a distance from the die holes and lag slightly, and defor-
mation is completes in 1.0 s. At the temperature of 1493 K,
the maximum stresses of the flat-die and cone-die are
95 MPa and 90 MPa, respectively. Figure 9g, h shows the
velocity change trend; the metal flow of the cone-die is
more stable than that of the flat-die.
3.1.2 Extrusion force changes with temperature and speed
Figure 10a shows extrusion force curves of the cone-die
at different speeds at a temperature of 1473 K. The
extrusion force remains unchanged as speed increases, the
breakthrough extrusion force values are 36.75 MN, 36.31
MN, 38.72 MN, 37.57 MN, 36.14 MN, and 35.69 MN at
speeds of 150 mm/s, 200 mm/s, 250 mm/s, 300 mm/s,
350 mm/s, and 400 mm/s, respectively. The extrusion
force values are all lower than 45 MN, and the extrusion
time obviously reduces as the speed increases. When the
speed exceeds 250 mm/s, the breakthrough extrusion force
begins to decrease, because one part of the kinetic energy is
converted into heat. Thus, deformation resistance decrea-
ses. This phenomenon can result in coarse grains or even
defects, so the extrusion speed of 304L H-shaped stainless
steel should be maintained below 250 mm/s.
Figure 10b shows extrusion force curves of the cone-die
at different temperatures at a speed of 250 mm/s. The
extrusion force decreases as temperature increases. The
breakthrough extrusion force values are 47.44 MN, 45.09
MN, 41.54 MN, 39.05 MN, and 36.31 MN at temperatures
of 1453 K, 1463 K, 1473 K, 1483 K, and 1493 K,
respectively. To maintain the breakthrough extrusion force
under 45 MN, the temperature should be higher than
1473 K.
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61 Page 4 of 10 L.-F. Tuo et al.
3.2 Extrusion production
3.2.1 Glass pad
Figure 11a, b shows the tool and glass pad. The material
of tool is plastic. The material of the glass pad is a mixture
of glass powder, sodium silicate and water, which is heated
to 473 K and retained for 2 h in a resistance furnace. When
the extrusion deformation of 304L H-shaped stainless steel
begins, the glass pad is heated rapidly, the glass softens
into a viscous state with the extrusion force and heat, and
glass adheres to the surface of the metal to form a glass
film. The glass film is driven out of the extrusion die hole
with the metal and provides lubrication. A thin, black layer
of glass film adheres to the surface of the H-shaped
stainless steel.
3.2.2 Heating curves
Figure 12a shows the heating curve of the circular fur-
nace, which is divided into five zones. ‘A1’ is the pre-
heating zone, and the target temperature is 1023 K. ‘A2’,
‘A3’, and ‘A4’ are the heating zones. The time in each zone
is 0.5 h, and the target temperatures are 1073 K, 1133 K,
and 1223 K, respectively. ‘A5’ is the soaking zone; the
target temperature is 1223 K and the time is 1 h. Fig-
ure 12b shows the heating curve of the induction furnace,
which is divided into four zones. ‘B1’ is the heating zone;
the target temperature is 1493 K and the heating power is
500 kW. ‘B2’ and ‘B3’ are the heating preservation zones;
the powers are 250 kW and 180 kW, respectively, at the
temperature of 1493 K. In order to make up for the tem-
perature drop, ‘B4’ is added, which is the final heating
zone. In this zone, the temperature rapidly rises to 1533 K
with the power of 550 kW.
Fig. 6 Metal flow
characteristics of the a flat-die
and b cone-die
Fig. 7 Transitive characteristic
of the force: a flat-die and
b cone-die
Fig. 8 Five trace points and locations
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Extrusion process of 304L H-shaped stainless steel used in passive residual heat removal heat… Page 5 of 10 61
3.2.3 H-shaped stainless steel products and extrusion
force curve
Based on the simulation results, the 304L H-shaped
stainless steels are manufactured using a Germany SMS 60
MN horizontal extruder, at a temperature of 1473 K and a
speed of 250 mm/s.
Figure 13a shows three different production experi-
ments. In failure case ‘I’, a flat-die (Fig. 3c) is adopted, the
inner shape of the cushion is a circular hole, and the flat-die
Fig. 9 (Color online) a Strain
of flat-die, b strain of cone-die,
c strain rate of flat-die, d strain
rate of cone-die, e stress of flat-
die, f stress of cone-die,g velocity of flat-die, and
h velocity of cone-die
123
61 Page 6 of 10 L.-F. Tuo et al.
Fig. 10 (Color online)
a Extrusion force changes with
temperature and b extrusion
force changes with temperature
speed
Fig. 11 a Tool and b glass pad
Fig. 12 (Color online)
a Heating curve of circular
furnace and b heating curve of
induction furnace
Fig. 13 (Color online)
a Production experiments,
b 304L H-shaped stainless steel,
and c surplus metal
123
Extrusion process of 304L H-shaped stainless steel used in passive residual heat removal heat… Page 7 of 10 61
and cushion all cracked. In failure case ‘II’, a flat-die is
adopted, the inner shape of the cushion is H-shaped
(Fig. 3b), but the inner shape of the glass pad is a circular
hole. In successful case ‘III’, the cone-die (Fig. 14a) is
adopted, and an H-shaped hole is designed in the cushion
and glass pad, as shown in Figs. 3b and 11b. Figure 13b, c
shows the successful 304L H-shaped stainless steel and
surplus metal, respectively.
The 304L stainless steel circular billet is successfully
deformed to H-shaped stainless steel at a temperature of
1473 K and a speed of 250 mm/s. Figure 14a shows the
cone-die after extrusion. Figure 14b shows the extrusion
force curve, which is divided into three stages. ‘C1’ is the
breakthrough phase, with a peak value of about 47 MN.
‘C2’ is the stable phase, with a mean value of 40 MN,
which is close to the simulated result. ‘C3’ is the unloading
phase, with a peak value of 42 MN. The extrusion force
increases slightly when extrusion stops.
Figure 15a shows the 304L H-shaped stainless steel
after the pickling process. The pickling solution is a mix-
ture of nitric acid (20%), hydrofluoric acid (10%), and
water (70%). Figure 15b shows the dye penetration test
after polishing according to the standard of RCC-M
MC4000. Figure 15c shows the final products after fixed
length. Figure 15d shows the welding assembly process of
the support frame. These 304L H-shaped stainless steels
were successfully used as the support frame of the PRHR
HX in the Haiyang nuclear power plant.
3.2.4 Mechanical properties and microstructures
Figure 16a shows the sampling location of 304L
H-shaped stainless steel according to GB/T 2975 (corre-
sponding to ISO 377: 1997). Samples 1# and 2# were cut
from two different 304L H-shaped stainless steels, which
were selected according to the Heat No. The Heat Nos. of
Sample 1# and Sample 2# are B3102498 and B3604718,
respectively. Figure 16b, c shows equiaxed austenite
microstructures of 304L H-shaped stainless steel. The grain
size is tested according to the ASTM E112-2013 standard.
Refining grain size is one method to improve the strength
of austenite stainless steels. Dynamic crystallization,
metadynamic crystallization, and grain growth occurred
successively during the extrusion process. The 304L
H-shaped stainless steel is rapidly deformed by the extru-
sion process. The extrusion time is usually no more than
10 s at the high temperature, after which the hot-state 304L
H-shaped stainless steel is annealed quickly in the cooling
water. In order to guarantee the strength of the H-shaped
stainless steel listed in Table 2, the grain size grade should
not be smaller than four. Ravi Kumar [14] studied the
effect of grain microstructure on the tensile properties and
stress–strain curve of 304L. Mirzadeh [15] studied the hot
deformation behavior of AISI 304L austenitic stainless
steel. After the solution annealing process, the fine grain
size can obviously improve the mechanical properties.
Table 2 shows the concrete values of solution annealed
mechanical properties and grain size grade. All mechanical
properties and microstructure meet the requirements of
application performance.
4 Conclusion
Analysis of the characteristics of metal flow, force
transitive on the die, and production experiments show that
the cone-die is favorable for metal flow and for decreasing
the extrusion force. A speed less than 250 mm/s can
decrease the risk of product defects, and a temperature
greater than 1473 K can guarantee that the extrusion force
remains under 45 MN. A U388-mm circular forging billet,
U400-mm extrusion container, circular furnace, and an
induction furnace can satisfy the dimensions of the 304L
H-shaped stainless steel, cushion, and glass pad with an
H-shaped hole and ensure that the metal flows out smoothly.
After pickling and polishing, the surfaces of the 304L
H-shaped stainless steel products pass the dye penetration
Fig. 14 a Cone-die and
b extrusion force curve
123
61 Page 8 of 10 L.-F. Tuo et al.
test. All mechanical properties and microstructures also
meet the requirements of application performance.
Acknowledgements The research presented in this paper is partially
funded by the Stainless steel tube branch company, Taiyuan Iron &
Steel (Group) CO. LTD. The 304L stainless billet and extrusion
productive experiment condition are provided by them. The authors
also thank laboratory engineer Xiao-Wen Zhang for his assistance in
conducting the tensile tests and metallographic test at the Technology
Center Laboratory, Taiyuan Iron & Steel (Group) CO. LTD.
Fig. 15 (Color online)
a Surface after acid pickling,
b dye penetration test, c final
products, and d manufacture of
the support frame
Fig. 16 a Sample location, b microstructure of Sample 1#, and c microstructure of Sample 2#
Table 2 Mechanical properties of 304L H-shaped stainless steel
Mechanical properties Yield strength Tensile strength Brinell hardness Total elongation Reduction of area Grain size number
Requirement C 205 MPa C 515 MPa B 92 C 30% C 40% C 4
Sample 1# 285/287 625/620 84.5/87/86 60/63 76/74 6/5
Sample 2# 289/295 645/635 85.5/87/87 62/62 72/75 6/6
123
Extrusion process of 304L H-shaped stainless steel used in passive residual heat removal heat… Page 9 of 10 61
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